1. Field of the Invention
The present invention generally relates to a technology for demodulating received signals, and particularly relates to a receiver apparatus based on an orthogonal frequency division multiplexing (OFDM) system.
2. Description of the Related Art
In the broadband wireless communications (next-generation mobile communications) that are currently researched in the technical field as identified above, there is a need to take into account multipath propagation environment when constructing a system. The multicarrier modulation method utilizes a plurality of subcarriers for a predetermined transmission band to transmit signals in parallel, thereby reducing the effect of frequency-selective fading, which becomes a problem especially in the multipath propagation environment. The OFDM system inserts a guard interval (GI) between effective symbols. This serves to effectively suppress inter-symbol interference with respect to multipath delayed waves that have delays shorter than the length of the guard interval, thereby providing for demodulation without equalization. Multipath fading can thus be effectively dealt with.
The delay (delay spread) of a delayed wave relative to a preceding wave increases depending on the communication environment. Such delay could be 0.2 through 2.0 microseconds in urban areas. In mountain areas or valleys, the delay could reach 10 through 20 microseconds, for example. In consideration of this, the guard interval should be set to such a sufficiently long length that all the delayed waves following the preceding wave fall within the range of the guard interval.
Since the guard interval is also a redundant symbol, however, it is preferable to avoid a drop in transmission efficiency while keeping the sufficient length of the guard interval. To this end, the length of the entirety of OFDM symbols needs to be increased so as to keep a ratio of the effective symbols to the guard interval above a certain level. If the length of OFDM symbols is increased, however, fading becomes inconstant within the duration of a single OFDM symbol, which results in weaker resiliency against fading. Further, an increase in the length (Ts) of an OFDM symbol results in a decrease in the subcarrier interval (Δf=1/Ts). This means weaker resiliency against a Doppler shift. Further, a ratio of a peak to an average power also increases, resulting in the degradation of performances caused by nonlinear distortion. In consideration of this, the guard interval is generally set to a decent length, and another measure is taken to deal with delayed waves arriving with delays exceeding the length of the guard interval.
In order to suppress inter-symbol interference (ISI) affecting the entirety of the utilized band, Non-patent Document 1 performs filtering in the time domain at a portion that triggers interference at the time of FFT (Fast Fourier Transform) during demodulation, thereby performing maximum likelihood sequence estimation (MLSE). A method of this kind is also described in Patent Document 1.
[Patent Document 1] Japanese Patent Application Publication No. 2003-218826
[Non-patent Document 1] Suyama, et al., “An OFDM Receiver with Smoothed FFT-Window and RLS-MLSE for Fast Multipath Fading Environments with Large Delay Spread”, IEEE 7th Int. Symp. on Spread-Spectrum Tech. & Appl., Prague, Czech Republic, Sep. 2-5, 2002, pp. 353-357
The methods as disclosed in Patent Document 1 and Non-patent Document 1 require a Viterbi equalizer that has M2 states for each subcarrier (M: the number of modulation levels). Because of this, these methods are not preferred from the viewpoint of a need to reduce circuit size, computation size, power consumption, costs, etc. Especially, these methods are disadvantageous for use in portable communications equipment that is required to be small.
Further, a method of this kind makes a hard decision with respect to signal points by use of maximum likelihood sequence estimation (MLSE) With respect to the certainness of signal points, therefore, likelihood information or soft decision information is not effectively utilized. Accordingly, such method fails to sufficiently utilize error correction techniques, and is disadvantageous from the viewpoint of the accuracy of received signal estimation.
Accordingly, there is a need for a receiver apparatus that can reduce inter-symbol interference (ISI) caused by delayed waves arriving with delays exceeding the guard interval of OFDM symbols.